ADD x. called a direct ADD, the byte to be ADDed is contained in the instruction.
ADD (x). called an extended ADD. this ADDs the byte found at memory location x.
ADD x,y. called an indexed ADD. this takes x and adds y to it then references the memory location of the sum, retrieves the byte and ADDs it to the ALU.
there is also the ADC instruction that adds the two bytes along with the carry from the last operation. this allows multibyte numbers to be added. the format is the same as ADD.
in all cases the result of the ADD is stored in the ALU.
ADD also refers to Attention deficit disorder.